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Stress relaxation in high polymers Pattison, James Parker

Abstract

The prediction of dynamic energy losses from stress relaxation data, using the equation ηω = - π/4.606 ( slope of relaxation curve), ……. (1) has received considerable attention recently, but for polyiso-butylene the correlation between predicted and experimental values of energy loss has been poor. This lack of agreement may have been caused by the failure of' the simple "step" distribution function of relaxation times to explain the process adequately. In this work, stress relaxation measurements with the RCA 5734 electronic transducer tube have been made in polylsobutylene at times as early as .01 second and the results show a very rapid rise in stress as t →0 rather than the constant value required by the "step" function. Another distribution function may be obtained by measuring the slopes of the log. dynamic modulus vs. log. ω , and the static modulus vs. log. time curves. For PIB the absence of data in the region near 1 second leaves a gap of nearly 2 cycles of log. time for which there are no predicted values of the distribution function and the data obtained in this research may be used to fill this gap. Simplified calculations with this function show qualitative agreement between predicted and experimental energy loss in polylsobutylene. Dynamic data for Acetate rayon show an Increase in energy loss with increased humidity, and stress relaxation experiments were performed on this material at various humidities to see if the effect was apparent. From equation (1) an increase in slope of the relaxation curve is expected to occur as the humidity is increased. No indication of this effect was found in the stress relaxation data, and in fact, the relaxation curves at higher humidity had slightly lower slopes. This would; tend to indicate that the prediction of energy loss from equation (1) is not valid in the case of Acetate rayon. Values of energy loss for Viscose rayon and Egg Albumin fiber at 65% humidity show moderate agreement with those calculated from stress relaxation experiments.

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